In recent years, an improved vacuum servo unit for a vehicle braking system has been developed in which an electrically activated solenoid is used for establishing an automatic braking operation, irrespective of a driver's brake pedal depression, when an emergency situation occurs such as the detection of a near obstacle in front of the moving vehicle. Such a vacuum servo unit can be applied to maintain a predetermined distance between adjacent vehicles and to maintain a vehicle on an upward slope.
A vacuum servo unit for a vehicle braking system similar to that described above is disclosed in German Patent Specification No. DE 4405092C1 which was published on Feb. 17, 1994. In the power piston assembly of this vacuum servo unit, an input member to be operated through operation of the brake operation member via an input rod is surrounded by a plunger which is displaced in the axial direction by energization of a solenoid. Sometimes the input rod may be inclined or slightly swung when the brake operation member is operated, and such an inclination urges the plunger in its radial direction. Since the resulting inclination of the plunger causes the plunger to be brought into engagement with one or more other neighboring elements which prevents the smooth movement of the plunger, a pipe shaped member is disposed between the power piston and the plunger. The reason for this is that in the absence of the pipe shaped member the friction between the plunger and each neighboring element prevents the plunger from being moved in the axial direction.
However, the provision of the pipe shaped member means that the number of parts comprising the unit increases as does the complexity of the unit. Also, the ease with which the power piston can be assembled is reduced.